The practice of so-called gravel packing a well in order to substantially block the flow of formation particulates into a wellbore while permitting the free flow of formation fluids has long been used. In such a technique, a particulate material is injected between the earth formation and a point in the wellbore. The particle size range of the particles is preselected and the particles are injected in such a manner that the packed particles will allow the flow of the desired fluid (the term being used to include liquids and/or gases) between the formation and the wellbore.
In carrying out a typical wellbore packing operation, a screen is first placed at a position in the wellbore which is within the desired fluid-bearing formation. In completed wells, a perforated steel casing is usually present between the screen and the formation. A slurry of particulate material in a carrier liquid is then pumped into the wellbore so as to place the particulate material between the screen and the casing (or the formation if no casing is present), as well as into the perforations of any casing, and also into any open area which may extend beyond the perforated casing into the formation. Thus, the aim of packing in most cases is to completely fill the area between the screen assembly and the formation with the particulate material. In some cases, this open area is packed with particulate material before placing the screen in the well. Such a technique is often referred to as prepacking such as described in U.S. Pat. No. 3,327,783.
The particulate material is typically gravel having a density (D.sub.p) of about 2.65 grams per cubic centimeter (g/cm.sup.3). The carrier liquid is generally water with a density (D.sub.c) of 1 g/cm.sup.3. The gravel particle size range is generally 20 mesh (all mesh sizes, U.S. mesh unless otherwise specified) to 40 mesh (841 microns to 420 microns) or 40 mesh to 60 mesh (420 microns to 250 microns). The resulting density ratio of particulate material to carrier liquid (D.sub.p /D.sub.c), is about 2.65/1.
many cases the overall packing efficiency (the percentage of the total volume of all of the area between the screen and the formation that is filled with gravel) is less than 100%. This is particularly true for deviated wells and especially for highly deviated wells (those deviating from the vertical at an angle more than 45 degrees) and horizontal wells. Of course, the lower the packing efficiency, the greater the likelihood of low production of well fluids or low injection rates and/or sand movement from the formation into the wellbore and production string.
In recent times, such as in copending U.S. application Ser. No. 905,355 filed Sept. 9, 1986, now abandoned, it has been suggested that the efficiencies of gravel pack operations can be improved by the use of particulate materials and carrier liquids with more closely matched density (D.sub.p /D.sub.c approaching 1/1), particularly in deviated wellbores. This technique results in improved packing efficiencies for, particularly, the upper portions of the packed interval as well as the top perforations in a highly deviated or horizontal wellbore since the particles are essentially neutrally buoyant in the carrier liquid and thus are transported more efficiently to the top portions of a wellbore and perforations. However, the cost of these specialized materials greatly exceeds the cost of simple sand packing materials.